Power supply apparatuses that include a main power supply and a backup power supply are typically used as power supply apparatuses to supply a power to information processing apparatuses. Power supply apparatuses convert an alternating current to a direct current and supply it to information processing apparatuses i.e., load circuits. The main power supply supplies electrical power when an AC (Alternating Current) power supply is in operation, whereas the backup power supply supplies electrical power when the AC power supply is stopped due to a power failure, for example.
Because the information processing apparatuses perform arithmetic processing during normal operation, load consumption currents thereof are large. Because the main power supply supplies electrical power during such normal operation, the capacity thereof needs to be large.
In contrast, when the AC power supply is stopped due to, for example, a power failure, the information processing apparatuses shift to a backup state. In the backup state, the arithmetic processing is not performed, and an operation that uses a minimum of power, such as memory retaining, is performed. These functions of reducing electrical power consumption and retaining data in this way when a power failure occurs are called a battery backup function or the like.
With a large-capacity power supply such as the main power supply, if the load current is small, the power conversion efficiency is reduced. Accordingly, when the information processing apparatus is in a backup state and electrical power consumption thereof is reduced, the conversion efficiency of electrical power increases if a lower-capacity backup power supply is used; therefore, it is possible to reduce the overall electrical power consumption by reducing the electrical power loss.
However, if electrical power is supplied from the backup power supply to the information processing apparatus during normal operation, there is a possibility that the backup power supply may go down due to, for example, abnormal temperatures because the power supply does not meet demand. Accordingly, when the information processing apparatus is in normal operation, electrical power needs to be supplied from the main power supply.
In contrast, if the backup power supply is stopped when the information processing apparatus is in normal operation, even when an abnormality occurs in the backup power supply, the abnormality cannot be detected. Because the backup power supply is needed at a time of emergency, such as a power failure, it is too late if an abnormal operation is detected only after starting the backup power supply.
Accordingly, power supply apparatuses are typically configured such that the voltage of the main power supply is set higher than that of the backup power supply, and, when the information processing apparatus is in normal operation, electrical power is supplied from the main power supply to the information processing apparatus while operating the backup power supply.
The conventional technologies as described above are disclosed in for example Japanese Laid-open Patent Publication Nos. 07-336915, 07-15888, and 09-37486.
In recent years, because the voltages of large scale integrated circuits (LSI) have been lowered, load voltages of information processing apparatuses are now set to +1.2 V or +1.8 V. Even when an attempt is made to set the voltage of the main power supply higher than that of the backup power supply, the allowable range of the operation voltage of the information processing apparatus or the setting range of the output of the power supply is small, which means high accuracy is required. Accordingly, there is a problem in that setting becomes difficult.
To prevent power from being supplied by the backup power supply when the information processing apparatus is in normal operation, the difference in voltage between the main power supply and the backup power supply needs to be set in such a manner that the output of the main power supply does not interfere with the output of the backup power supply, i.e., the two voltages do not match. For example, if the load voltage is +1.2 V and the output accuracy of the power supply is ±5%, to avoid interference even when the output of the main power supply falls by 5% and the output of the backup power supply rises by 5%, a potential difference of 10% or more is needed. Accordingly, when the output voltage of the main power supply is 1.2 V+0.12 V=1.32 V, the output voltage exceeds the load voltage allowable range, i.e., 1.2 V±5% (1.14 to 1.26 V).
In this way, by setting a voltage difference between the main power supply and the backup power supply, the conventional technology for preventing electrical power from being supplied from the backup power supply to the information processing apparatus that is in normal operation cannot be used if the information processing apparatus load voltage is low, which is a problem.